Examiners Report June GCSE Chemistry 5CH3H 01

Transcription

1 Examiners Report June 2016 GCSE Chemistry 5CH3H 01

2 Edexcel and BTEC Qualifications Edexcel and BTEC qualifications come from Pearson, the UK s largest awarding body. We provide a wide range of qualifications including academic, vocational, occupational and specific programmes for employers. For further information visit our qualifications websites at or Alternatively, you can get in touch with us using the details on our contact us page at Giving you insight to inform next steps ResultsPlus is Pearson s free online service giving instant and detailed analysis of your students exam results. See students scores for every exam question. Understand how your students performance compares with class and national averages. Identify potential topics, skills and types of question where students may need to develop their learning further. For more information on ResultsPlus, or to log in, visit Your exams officer will be able to set up your ResultsPlus account in minutes via Edexcel Online. Pearson: helping people progress, everywhere Pearson aspires to be the world s leading learning company. Our aim is to help everyone progress in their lives through education. We believe in every kind of learning, for all kinds of people, wherever they are in the world. We ve been involved in education for over 150 years, and by working across 70 countries, in 100 languages, we have built an international reputation for our commitment to high standards and raising achievement through innovation in education. Find out more about how we can help you and your students at: June 2016 Publications Code 5CH3H_01_1606_ER All the material in this publication is copyright Pearson Education Ltd GCSE Chemistry 5CH3H 01

3 Introduction Paper 5CH3H is taken by candidates for GCSE Chemistry and for GCSE Further Additional Science. It follows the usual format of six questions. There is a mixture of multiple choice items, short answer items and two extended response (6 mark) items. There was a good response to this paper overall, with the strongest candidates answering the trickier parts well calculations and equations but also the ionic equation and the structural formula of the ester, for example. Question 6(d) produced many excellent answers with well argued, logical approaches to the organic identification. GCSE Chemistry 5CH3H 01 3

4 Question 1 (b) Most candidates gained at least 1 mark on this question and many gained both. The most common correct response was boiling the water or evaporation, and the subsequent production of limescale. Some candidates failed to score the second mark, e.g. by forming a precipitate following boiling or using scum instead of scale. Some candidates' responses included the use of soap solution, but the main error on the part of those who had read the question involved the inappropriate use, in this context, of sodium hydroxide solution or other ion tests. Question 1 (c) This question was generally well answered, with the vast majority naming an ion exchange column or resin and correctly identifying some or all of the ions involved in the exchange. A few, instead of 'ions' being exchanged, used elements or atoms or molecules or compounds, or did not remember which ions were exchanged. The addition of sodium carbonate or distillation of the water were rarely seen responses, although some candidates did suggest boiling as an appropriate response and were confused about its role in water softening. Examiner Comments An example of a good response. Examiner Tip This question says "Explain..." so, like this candidate, identify the particles being exchanged as ions to give a correct explanation. 4 GCSE Chemistry 5CH3H 01

5 Question 1 (d) A majority of candidates discussed problems with lathering and the need for more soap (and the additional cost of this). Unfortunately, some stated that, since hard water made it more difficult to form a lather, more water is needed to form a lather ( water is wasted ). Fewer mentioned the formation of scum. Although some candidates referred to limescale there were fewer references to blocked pipes or kettles or other appliances and the consequent reduction in their efficiency. A small minority of candidates believed that hard water is caused by the presence of aluminium ions and gave its associated disadvantage as contributing to the cause of Alzheimer s disease. Other incorrect ideas included hard water being unhealthy, rough on the skin or having a bad taste. It is important that the distinction between scum and limescale is known, and also that in a question such as this the cause and the consequent effect is given. Examiner Comments This candidate has made valid points but the answer is not clearly expressed. Examiner Tip Be clear in your answer: More of what is used? Why is limescale a problem? GCSE Chemistry 5CH3H 01 5

6 Examiner Comments This answer contrasts with the other example. Examiner Tip This candidate has explained the problem: Problem: limescale Explanation: damages washing machines Problem: uses more soap Explanation: costs more money Question 2 (a) (i) The vast majority of candidates gained both marks here. Where marks were lost it was either due to an incorrect colour of the precipitate (usually cream) or stating additional phenomena (e.g. fizzing). 6 GCSE Chemistry 5CH3H 01

7 Question 2 (a) (iii) The difference between qualitative and quantitative is generally very well known. Many candidates gave blood analyses as examples. Some mistook qualitative tests as tests for quality or purity. Examiner Comments Read through your answers when you have finished. This answer has used qualit(at) ive twice in the first sentence, and so has contradicted itself. Examiner Tip The use of examples in this case alcohol levels in blood can be a good idea, but unfortunately this is a quantitative test. GCSE Chemistry 5CH3H 01 7

8 Question 2 (b) There was a good response to this question, with many answers awarded all the marks. Those candidates who scored zero did so as a result of using an incorrect reagent, flame tests or by giving the test for iodide ions. Occasionally the use of litmus was recommended. There was good chemistry seen and often (as a bonus) correctly explained in such responses. The main reason for the loss of 1 mark was the failure to use excess of sodium hydroxide solution. Examiner Comments A well described and explained answer. (The candidate should have referred to sodium hydroxide solution, but this was not penalised). 8 GCSE Chemistry 5CH3H 01

9 Question 3 (a) Many candidates gained 1 mark for calculating M r (NaOH) as 40, but then went on to work out the M r of ammonia as well. An inverted calculation of 40/4 was often seen. Numeracy skills are at a premium when allied to chemistry (or otherwise). Many candidates just added up all the masses or worked on ammonium chloride instead of sodium hydroxide. Of those that did progress, the main error was to fail to multiply by the mole volume of the gas (24 dm 3 ). Examiner Comments This calculation is set out clearly for the examiner to follow. Whilst the answer is correct, so scores full marks, had there been an error it would have been straightforward to award part marks. Examiner Tip Set out working clearly. This answer clearly shows: 1:1 mole ratio Mr of NaOH and subsequent mole calculation of 0.1 moles volume = moles x 24 GCSE Chemistry 5CH3H 01 9

10 Examiner Comments This answer was incorrect but was awarded part marks because the working was clear enough. Question 3 (b) (i) There were many perfect answers here, although it was noted that some candidates did not notice that the question was worth 2 marks and merely stated that a dynamic equilibrium is a reversible reaction with nothing further written. Some candidates used poor terminology, such as reactions cancel each other out' it is important that correct scientific explanations are given. A reasonably common misconception was that, at equilibrium, the amount of reactants equals the amount of products. 10 GCSE Chemistry 5CH3H 01

11 Question 3 (b) (ii) This question proved challenging to some candidates. Many wrote about compromise and then related their responses to danger and/or expense rather than the consequence of temperature change in terms of the position of equilibrium. Thus, some were able to state that a higher temperature would reduce the yield of ammonia, but only a minority of these could explain this in terms of the higher temperature favouring the endothermic reaction (or vice-versa). There were even some comments about denaturing enzymes at 450 o C. Examiner Comments There is no need to mention rate it is in the question but this is a good answer. Question 3 (c) This item was surprisingly poorly answered. Many students did not understand the ionic basis of acid-alkali reactions, so many responses gave the name of a compound, rather than an ion. Examiner Comments An incorrect formula for hydroxide. Examiner Tip Ions must have charges! GCSE Chemistry 5CH3H 01 11

12 Question 4 (b) (i) Many candidates did not fully direct their answer to the question the use of this apparatus. Most candidates scored 2 or 3 marks, forgetting either the DC supply and/or the naming of a suitable electrolyte. There were some unnecessary (albeit usually correct) descriptions of oxidation and reduction and/or movement of ions, discussing the process, rather than the use of the apparatus. Some candidates who correctly described the electrodes then did not go on to describe an electrolyte, or suggested incorrect electrolytes such as copper hydroxide or hydrochloric acid. Several candidates thought that the anode attracted impure copper and that the electrolyte was impure copper. Some confused the anode with the cathode. Some candidates annotated the diagram and were credited. Question 4 (b) (ii) Many candidates recognised that the deposit (often called sludge) was due to impurities; some described how the impurities had no charge so were not attracted to an electrode. A few did refer to the impurities containing gold or silver. Fewer scored both marks, the candidates not explaining that the impurities were released when the copper ions were removed from the anode. There was evidence of a lack of understanding with "impure copper atoms" or ions, whatever such a particle might be. Hence, some answers said that the deposit was impure copper rather than the impurities themselves. Some even suggested that it was electrons that were deposited under the anode. Question 4 (c) (i) Most candidates knew about reduction, but fewer stated that it was due to the gain of electrons. Some stated that lead (which is the product) gained electrons rather than lead ions, an understandable error, but one that must be avoided. Some candidates described the wrong species being reduced (some form of bromine or even copper). Examiner Comments This example illustrates the need to describe the particles correctly. Examiner Tip Refer to the species as atoms, ions etc. In this case, lead ions are reduced to form lead atoms. 12 GCSE Chemistry 5CH3H 01

13 Examiner Comments This answer contrasts with the other example. Question 4 (c) (ii) Ionic equations are difficult and some candidates answered this question incorrectly. On the left, bromine atoms and Br 2 ions appeared in place of bromide ions. On the right, Br 2 or bromide ions appeared. GCSE Chemistry 5CH3H 01 13

14 Question 5 (a) (i) There is a misunderstanding of the formation of esters and their structure/formula(e). The product given was often a (mangled) carboxylic acid or alcohol. Sometimes not all the H atoms were accounted for or an extra O was given, possibly by candidates not noticing that water was given as one of the products. Too many formulae had incorrect bonds and/or functional groups, with penta and even hexa-valent carbon as well as hydrogen in the middle of carbon chains. The correct molecular formula was sometimes seen. Question 5 (a) (ii) There were many correct responses seen, including 'dissolving' which was allowed. Too many lists were present, when an incorrect answer cancelled out a correct one, especially precipitates of many hues, and also colour changes and confusion with limewater going cloudy. Question 5 (c) This question was mostly high scoring, with some excellent and accurate descriptions of titration, with many knowing the correct indicator to use. Many knew the importance of obtaining concordant results. Some said that they would filter the salt before evaporation. Crystallisation was less well covered, although most managed at least a basic description of getting a salt (by evaporation). However, only a few mentioned heating until incipient crystallisation occurs, by reaching a saturated solution, followed by leaving to crystallise. With crystallisation, there were good references to mixing the acid and alkali without the indicator. Weaker candidates did not recognise that sodium ethanoate was in aqueous solution, and gave a description of precipitation and/or the washing of crystals in distilled water before final drying. Suggestions for teaching: Pipette was correctly named (and spelled), but the burette less so The volume of alkali added to a flask was often omitted Sometimes a specific volume of acid was added to the burette, rather than just filling There is still a desire to use Universal Indicator Some got the colours for the indicators incorrect Some students discussed precipitation It is recommended that all candidates should have the opportunity to carry out or at least observe this experiment 14 GCSE Chemistry 5CH3H 01

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16 Examiner Comments A good example of a six mark answer. 16 GCSE Chemistry 5CH3H 01

17 Examiner Comments This scored six although it is not perfect and is included for teachers to use with their classes. GCSE Chemistry 5CH3H 01 17

18 Question 5 (d) Some candidates found this calculation difficult. Many did not recognise the 1:1 ratio, and instead discussed a 1:2 ratio leading to an incorrect response of 50 cm 3. Many did not divide by 1000 to get the concentration of ethanoic acid correct. Some scored the mark for the correct ratio or for calculating the moles of acid, but were unable to complete the calculation. Examiner Comments An example of a correct answer. 18 GCSE Chemistry 5CH3H 01

19 Question 6 (b) (i) Nearly all of the candidates understood that whisky had a greater concentration of ethanol than beer. However, only some went on to clearly explain why this caused danger. Responses were sometimes vague about drunkenness or even that the more alcohol had a bigger effect. Some candidates confused reaction time with reaction speed. With alcohol, reaction times are longer or reaction speeds are slower. 'Slower reaction times was not accepted. Only a small number of candidates referred to poorer vision, dizziness or depressant effect. Examiner Comments This scored only 1 because reaction time is longer (or reaction speed slower) is the correct response. Question 6 (b) (ii) Most candidates scored this mark, although some candidates did not know what a homologous series was. Question 6 (c) The formula for propene was not known by all, and some put water on the incorrect side of the equation (with some equations having H 2 O on the left hand side which was not accepted). Candidates must be more careful about using capital/small letters and using subscripts. Question 6 (d) This was a well answered question with many candidates awarded 6 marks, and the vast majority of candidates gaining at least 4. The main error was the confusion between alkanes and alkenes, which should have been revised for the C1 paper. There were also some contradictions in responses and candidates should be encouraged to re-read their work, particularly on 6 mark questions. Poor handwriting meant that on occasions it was difficult to discern the difference between ethane and ethene. One good approach was to identify the two hydrocarbons, and then distinguish them, followed by the two compounds containing oxygen. Many answers were communicated fluently and concisely, and candidates often appeared to enjoy solving the puzzle using their chemical knowledge. GCSE Chemistry 5CH3H 01 19

20 Paper Summary On the basis of performance in this paper, candidates are offered the following advice: Practise writing balanced equations, taking care with subscripts Practise, using past papers, examples of calculations In calculations lay out your answers step by step, making clear what has been evaluated in each step Be able to explain how changes in temperature and pressure affect the position of equilibrium Learn the ionic nature of acid-base neutralisation Practise drawing structures of organic molecules, particularly esters Learn the difference between heating to dryness and crystallisation. 20 GCSE Chemistry 5CH3H 01

21 Grade Boundaries Grade boundaries for this, and all other papers, can be found on the website on this link: GCSE Chemistry 5CH3H 01 21

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